Method for the electrolytic refining of copper

ABSTRACT

A copper electro-refining process is disclosed for producing heavy deposits of copper by utilizing a metal cathode, such as titanium, which is self-passivating in a copper sulfate-sulfuric acid electrolyte, the electro-refining process being carried out at relatively high current densities utilizing a periodic forward and reverse flow of current in which the forward current (that is, the plating current) is carried out for a time period ranging from about 10 seconds to 100 seconds at a relatively high current density, the reverse current being applied at a lower current density for a time ranging from about one-fifteenth to onefiftieth of the time employed during the forward flow of current.

United States Patent [1 1 Brytczuk et al.

[ Feb. 4, 1975 METHOD FOR THE ELECTROLYTIC REFINING OF COPPER [75] Inventors: Walter L. Brytczuk, Roselle Park;

Milton .1. Hauser, Scotch Plains,

21 Appl. No.: 371,876

[52] US. Cl 204/108, 204/3, 204/228,

[51] Int. Cl. C22d 1/16, C23b 7/00, BOlk 3/00 FOREIGN PATENTS OR APPLICATIONS 12,506 5/1968 Japan 204/108 Primary Examiner-John H. Mack Assistant Examiner-Aaron Weisstuch Attorney, Agent, or Firm-Kasper T. Serijan; Eugene J. Kalil [57] ABSTRACT A copper electro-refining process is disclosed for producing heavy deposits of copper by utilizing a metal cathode, such as titanium, which is self-passivating in a copper sulfate-sulfuric acid electrolyte, the electrorefining process being carried out at relatively high current densities utilizing a periodic forward and reverse flow of current in which the forward current (that is, the plating current) is carried out for a time period ranging from about 10 seconds to 100 seconds at a relatively high current density, the reverse current being applied at a lower current density for a time ranging from about one-fifteenth to one-fiftieth of the time employed during the forward flow of current.

5 Claims, N0 Drawings METHOD FOR THE ELECTROLYTIC REFINING OF COPPER efficiency of the electrolytic refining of copper at sub-- stantially high current densities.

STATE OF THE ART Commercially acceptable electrolytically refined copper is usually made by depositing copper onto a copper starter sheet of about 0.025 to 0.035 inch in thickness until anywhere from about 100 to 300 pounds of copper have been accumulated, depending upon the overall dimensions of the copper cathode. In one method, the starter sheets become part of the finished cathode and are melted therewith to form wire bars or other product. In another method, a heavier copper cathode blank is coated with a thin parting film of oil so that the deposited copper can be stripped therefrom.

Both of the above methods use unidirectional current flow to dissolve an impure copper anode and to deposit a relatively pure copper cathode.

The electrolytic refining process is usually carried out as a multiple process in a series of tanks; that is to say, each tank contains a plurality of copper starting sheets which are electrically coupled in parallel and alternately spaced between copper anodes which are similarly electrically coupled in parallel, the current flow in the tank being from the anodes to the cathodes. Thus, the individual electrodes in the tanks are connected electrically in parallel and the tanks are connected in series.

The starting sheets are generally flimsy and, during the electrolytic deposition of copper, warpage often occurs and, in order to avoid short circuiting in the plating bath, the cathodes are frequently removed and straightened. Frequent short circuiting is apt to occur which adversely affects the plating efficiency and which, therefore, must be avoided. 1

The current densities employed are limited by the tendency of the electrodes to passivate which interferes with the production of refined copper. The limit of the current densities varies with the impurities present and other operating conditions. Generally, the current densities range from to 30 amps/sq. ft.

Economic disadvantages of the foregoing process are the large amount of tankhouse labor expended with regard to cathode starting sheets, including the production of starting sheets by electrolytic deposition; the straightening, punching and the hanging of starting sheets from rods; the correction of warped and shorted cathodes, and the subsequent tending thereof during plating. It has been estimated that over 60 percent of the tankhouse labor involves the foregoing. Thus, if the aforementioned costly operations could be eliminated, great savings would result.

A method which has been proposed comprises using rigid reusable cathodes, such as copper, titanium, stainless steel and the like, which are fairly thick and, therefore. substantially free from warpage. The cathodes are generally provided with a parting material, such as oil, to enable the stripping of the plated copper from the rigid cathodes, which cathodes are then recycled back to the tank for the further deposition of copper thereon. While this was an improvement in some respects, the capital cost for equipping the tankhouse was so high that amortization and interest tended to offset savings in labor. Thought was then given to increasing productivity per tank by increasing the current density. However, even this had its limitations due to passivation of the electrodes. The voltage would rise and the refining process would thus be disturbed. Impurities, e.g., such as sulfates, Ag, Pb, Se, Te, As, and the like, tended to be occluded with the deposited copper which adversely affected the electrical and physical properties of wire bars and other shapes produced from the cathode deposits.

Recent attempts have been made to increase productivity by employing periodic reversal of current at relatively high current densities. One such method is disclosed in British Pat. No. 1,157,686. The copper is deposited on a starting sheet in which the ratio of the conducted charge in'the negative to positive direction at the cathode ranges from I021 to 40:], that is to say, the time period for the forward current (i.e., from anode to cathode) relative to the reverse current (i.e., from cathode to anode) is 10 to 40 times greater than the time period of the reverse current at current densities ranging from over 20 to about 65 amps/sq. ft.

However, while the foregoing was an improvement over prior methods, there were certain inherent disadting a limit on the increase in productivity per tank. In

addition, there was a tendency for the impurities in the bath to be occluded within the copper deposit.

We have found unexpectedly that by using a selfpassivating cathode metal in place of the copper starting sheet or the coated reusable blank, it is possible to use periodic reversal, higher current densities and reusable blanks to produce more copper per unit tank. In addition, the quality of the copper product is improved as evidenced by a decrease in deleterious impurities in the refined copper product.

OBJECTS OF THE INVENTION It is thus the object of the present invention to provide a method for the electrolytic refining of copper characterized by improved productivity and improved quality of product.

Another object is to provide a method for the electrolytic refining of copper using periodic reversal of current at relatively high current densities in combination with metal cathodes which are self-passivating relative to copper sulfate electrolytes to improve the refining of copper and to do it more economically both in capital investment and in operating costs.

These and other objects will more clearly appear from the following disclosure and the appended claims.

STATEMENT OF THE INVENTION One embodiment of the invention resides in an electrolytic process for producing heavy deposits of high purity copper on a cathode in which the selected cathode metal is self-passivating relative to copper sulfate electrolytes, the cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen at about 25C. Examples of cathodes which are useful in carrying out the invention are those selected from the group consisting of titanium, zirconium and stainless steel, among others, titanium being particularly preferred.

Following selection of the cathode material which is fabricated into a rigid reusable substantially nonwarpable cathode, a plating system is then established comprising a plurality of cathodes immersed in a copper sulfate electrolyte alternately spaced between anodes. The system is then continuously subjected to a periodic forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively, the time period for the forward flow of current being substantially longer than the time period for the reverse flow of current.

The forward flow of current (i.e., the plating current) is essentially carried out for a time ranging from about seconds to 100 seconds at a forward current density ranging from about 20 to 65 amps/sq.ft., the reverse flow of current being carried out for a time period ranging from about one-fifteenth to one-fiftieth of the time employed in the forward current flow. The current density of the reverse current flow is maintained at a level below the forward current density, the amount ranging from about to 65 percent less than the forward current density. The periodic application of the current to the system is maintained until a heavy deposit of strippable high purity copper is obtained on the cathodes.

We have found that by working over the foregoing parameters, optimum results are obtained. For example, if the forward flow of current at a relatively high current density is carried out for a time period substantially in excess of 100 seconds, there is a tendency for impurities in the bath to be occluded with the deposited copper. In addition, if the forward plating time is too long, the copper deposit may tend to polarize and adversely affect the current efficiency of the process. This is avoided by periodically reversing the current at lower current densities provided the forward flow of current does not substantially exceed 100 seconds during the plating cycle. The advantage of using a substantially lower current density during the reverse flow is to as sure optimum effective current efficiency during the process.

We have found, much to our surprise, that by using a self-passivating cathode metal, such as titanium, in place of the conventional copper starting sheet, markedly improved current efficiencies are obtained. This will be shown hereinafter.

While the time period for the forward flow of current may range broadly from about 10 to 100 seconds, we prefer to carry out the forward flow of the current or the plating part of the cycle over a time period of about 15 to 55 seconds at a current density of about 25 to 45 amps/sq.ft. Similarly, in the reverse cycle, we prefer that the time period of the reverse flow of current range from about one-eighteenth to one-twenty second of the time period employed in the forward flow of current, the current density during reverse flow ranging from about 25 to 45 percent less than the current density corresponding to the forward flow of current.

DETAILS OF THE INVENTION Tests were carried out in accordance with the invention using copper anodes measuring 36 inches by 36 inches and 1% inch thick, with copper starting sheets as the cathode on the one hand having substantially the same dimensions except for a thickness of about 0.025 inch and titanium cathode, on the other hand, having a thickness of about 0.12 inch. A typical tank or cell is adapted to hold 31 copper anodes and 32 cathodes, the anodes being supported between the cathodes. The total area of the cathode is in the --neighborhood of about 20 square feet. As illustrative of the invention, the following examples are given:

EXAMPLE 1 A plating system was prepared comprising 38 tanks connected in series with each of the tanks containing 31 copper anodes as aforementioned and 32 cathodes. Two of the tanks were set up using titanium cathodes, with the remaining 36 tanks set up for copper cathodes.

The copper sulfate electrolyte was circulated through each tank at a rate of4 gallons/minute per tank. A typical copper electrolyte contained about 172 grams/liter of H 42 grams/liter of copper, 0.017 gram/liter of Cl, about 10 grams/liter of Ni, 1.1 grams/liter of As and 0.7 grams/liter of Sb. The electrolyte also contained addition agents to the extent of 0.077 lb/ton glue and 0.42 lb/ton lignone. The temperature of the electrolyte varied from F to F.

The forward flow of current was 18,000 amps (at 0.37 to 0.43 volts/tank) and the reverse flow about 11,500 amps (at 0.16 to 0.21 volt/tank). The forward flow of current was applied periodically for 24 seconds at a current density of 30 amps/sq.ft., while the reverse flow of current was applied periodically between the forward flow for 1.2 seconds at a lower current density of 20 amps/sq.ft., that is, 33 percent less than the forward current density. As will be noted, the reverse current is applied for about 1.2/24 or one-twentieth of the time period of the forward flow.

After about 5.6 days of plating at a cycle efficiency of 95.6 percent,

[(for. curr.) (for. time) (rev. curr.) (rev. time)] (100) Cycle (for. curr.) (for. time) +(rev. ourr.) (rev. time) the following results were obtained:

Copper Star ting Sheets As will be noted from the foregoing data, under the same conditions of plating, the titanium cathodes provided markedly improved results over cathodes made of copper starting sheets. Thus, based on the average current efficiency, the titanium cathodes (87.8 percent efficiency) provided an increase over conventional copper cathode (77.2 percent efficiency) of about percent based on the copper cathode value. Even the average effective current efficiency of titanium is superior to copper. This value is obtained by multiplying the average current efficiency by the aforementioned cycle efficiency of 95.6 percent.

In addition to the foregoing differences, the tank with the titanium cathodes also exhibited a copper output of 798 lbs. per tank day obtained with the copper cathodes. Thus, the use of titanium cathodes provides an increase in copper output at the titanium cathodes of over 13 percent. This is an effective increase, considering that, in addition, the use of titanium cathodes also drastically decreases the amount of tankhouse labor required in working with copper starting sheets.

It should be borne in mind that the use of titanium cathodes coupled with the use of the periodic reversal of current also substantially doubles the output per tank compared to the conventional practice of using only the forward flow of current.

EXAMPLE 2 In another test run, the following copper sulfate electrolyte solution was employed: 177 grams/liter H 80 44 grams/liter of Cu, 0.019 gram/liter of Cl, 9.9 grams/- liter of Ni, 0.99 gram/liter of As and 0.55 gram/liter of Sb, the solution being circulated through the tank at 4 gallons per minute, the temperature ranging from about 142F to 148F. The addition agents added to the solution comprised 0.05 lb/ton of glue and 0.25 lb/ton of lignone.

As in Example 1, the forward flow of current was 18,000 amps (at 0.37 to 0.43 volt/tank) and the reverse flow about 11,500 amps (at 0.16 to 0.2 volt/tank). The forward flow of current was also periodically applied for 24 seconds at amps/sq.ft. and the reverse flow for 1.2 seconds at 20 amps/sq.ft., the total time being about 15.8 days. The cycle efficiency was calculated as 95.4 percent. Both copper starting sheets and titanium blanks were tested as the cathodes. The following results were obtained:

Copper Starting Sheets Average current efficiency 85.2%

Average effective current efficiency 813% Pounds of copper deposited per tank day 874 Titanium Blanks Average current efficiency 96.9

Average effective current efficiency 92.5

Pounds of copper deposited per tank day 990 Examples of other periodic cycles and current densities which may be employed in carrying out the invention are as follows:

EXAMPLE 3 In this run, the forward flow of current with respect to a titanium cathode was 21,000 amps (at about 0.45 volt/tank) and the reverse flow of current about 1 1,000 amps (at about 0.16 volt/tank). The forward flow of current was periodically applied for 30 seconds at 35 amps/sq.ft. and the reverse flow for 1.5 seconds at 18 amps/sq.ft., the total time being about 4.85 days. The cycle efficiency was calculated as 94.5 percent. The results obtained with the titanium blank used as the cathodes are as follows:

Titanium Blank Average current efficiency 95.4% Average effective current efficiency 90.5% Pounds of copper deposited per tank day 1125 Note the highly improved tank yield.

Generally speaking, the process is carried out until a heavy deposit of copper is obtained ranging from oneeighth to one-half inch thick on each face of the cathode.

Examples of other self-passivating cathode materials are zirconium and stainless steel, such as those referred to as 18/8 stainless.

Generally speaking, the copper sulfate electrolyte may range in composition from about to 240 grams/liter H 80, and from about 20 to 50 grams/liter of copper. As will be appreciated, such solutions may contain small amounts of other ingredients, such as Cl, Ni, Sb, As, among others. It is usually understood that such plating solutions contain small but effective amounts of addition agents normally employed to assure optimum quality of plating. it is understood that the process may be employed in the electrolytic refining of copper anodes or in the electrowinning of copper from copper electrolytes using insoluble anodes.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. An electrolytic process for producing heavy deposits of high purity copper on a cathode at improved production rates which comprises,

selecting a metal cathode material which is selfpassivating in a copper sulfate electrolyte,

said cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen at about 25C, providing a plating system comprising a plurality of said cathodes immersed in said copper sulfate electrolyte alternately spaced between anodes, continuously subjecting said system to a periodic forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively, said forward flow of current being carried out for a time ranging from about 10 seconds to 100 seconds, at a current density of about 20 to 65 amps/sq.ft., said reverse flow of current being carried out for a time ranging from about one-fifteenth to onefiftieth of the time employed in the forward flow of current, the reverse flow of current being maintained at a current density level of to 65 percent less than the current density corresponding to the forward flow of current, and maintaining said periodic application of the current to said system until a heavy deposit of strippable high purity copper is obtained on said cathodes. 2. A copper electro-refining process for producing heavy deposits of high purity copper on a cathode at improved production rates which comprises,

providing a metal cathode material selected from the group consisting of titanium, zirconium and stainless steel which is self-passivating in a copper sulfate electrolyte, said cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen at about 25C, providing a plating system comprising a plurality of said cathodes immersed in said copper sulfate electrolyte alternately spaced between copper anodes to be refined, continuously subjecting said system to a period forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively, said forward flow of current being carried out for a time ranging from about 10 seconds to 100 seconds, at a current density of about to 65 amps/sq.ft., said reverse flow of current being carried out for a time ranging from about onefifteenth to one-fiftieth of the time employed in the forward flow of current, the reverse flow of current being maintained at a current density of 15 to 65 percent less than the current density corresponding to the forward flow of current, and maintaining said periodic application of the current to said system until a heavy deposit of strippable high purity copper is obtained on said cathodes. 3. The copper electro-refining process of claim 2, wherein said cathode metal is titanium, wherein the time period during the forward flow of current ranges from about 15 to 55 seconds at a current density of about to 45 amps/sq.ft.,

wherein the time period during the reverse flow current is about one-eighteenth to one-twenty second of the time period employed in the forward flow of current, and

wherein the current density of the reverse flow of current is 25 to 45 percent less than the current density corresponding to the forward flow of current.

4. A copper electro-refining process for producing heavy deposits of high purity copper on a cathode at improved production rates which comprises,

selecting a metal cathode material which is selfpassivating in an electrolytic copper solution,

said cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen,

providing a plating system comprising at least one tank containing a plurality of said cathodes immersed in said copper sulfate electrolyte alternately spaced between copper anodes to be refined,

said copper electrolyte containing 20 to 50 grams/- liter of copper in the form of copper sulfate and to 240 grams/liter of sulfuric acid making up substantially the balance of said solution,

continuously circulating said electrolyte through said tank by means of a circulating system, continuously subjecting said system to a period forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively,

said forward flow of current being carried out for a time ranging from about 10 seconds to 100 seconds, at a current density of about 20 to 65 amps/sq.ft.,

said reverse flow of current being carried out for a time ranging from about one-fifteenth to onefiftieth of the time employed in the forward flow of current, the reverse flow of current being maintained at a current density level of 15 to 65 percent less than the current density corresponding to the forward flow of current,

and maintaining said periodic application of the current to said system until a heavy deposit of high purity copper is obtained on said cathodes.

5. The copper electro-refining process of claim 4,

wherein the cathode metal is selected from the group consisting of titanium, zirconium and stainless steel, wherein the time period during the forward flow of current ranges from about 15 to 55 seconds at a current density of about-25 to 45 amps/sq.ft.,

wherein the time period during the reverse flow current is about one-eighteenth to one-twenty second of the time period employed in the forward flow of current, and

wherein the current density of the reverse flow of current is 25 to 45 percent less than the current density corresponding to the forward flow of current. 

2. A copper electro-refining process for producing heavy deposits of high purity copper on a cathode at improved production rates which comprises, providing a metal cathode material selected from the group consisting of titanium, zirconium and stainless steel which is self-passivating in a copper sulfate electrolyte, said cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen at about 25*C, providing a plating system comprising a plurality of said cathodes immersed in said copper sulfate electrolyte alternately spaced between copper anodes to be refined, continuously subjecting said system to a period forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively, said forward flow of current being carried out for a time ranging from about 10 seconds to 100 seconds, at a current density of about 20 to 65 amps/sq.ft., said reverse flow of current being carried out for a time ranging from about one-fifteenth to one-fiftieth of the time employed in the forward flow of current, the reverse flow of current being maintained at a current density of 15 to 65 percent less than the current density corresponding to the forward flow of current, and maintaining said periodic application of the current to said system until a heavy deposit of strippable high purity copper is obtained on said cathodes.
 3. The copper electro-refining process of claim 2, wherein said cathode metal is titanium, wherein the time period during the forward flow of current ranges from about 15 to 55 seconds at a current density of about 25 to 45 amps/sq.ft., wherein the time period during the reverse flow current is about one-eighteenth to one-twenty second of the time period employed in the forward flow of current, and wherein the current density of the reverse flow of current is 25 to 45 percent less than the current density corresponding to the forward flow of current.
 4. A copper electro-refining process for producing heavy deposits of high purity copper on a cathode at improved production rates which comprises, selecting a metal cathode material which is self-passivating in an electrolytic copper solution, said cathode metal being characterized by a negative free energy of formation of the oxide of at least about 90,000 calories per gram atom of oxygen, providing a plating system comprising at least one tank containing a plurality of said cathodes immersed in said copper sulfate electrolyte alternately spaced between copper anodes to be refined, said copper electrolyte containing 20 to 50 grams/liter of copper in the form of copper sulfate and 100 to 240 grams/liter of sulfuric acid making up substantially the balance of said solution, continuously circulating said electrolyte through said tank by means of a circulating system, continuously subjecting said system to a period forward and reverse flow of current from said anodes to said cathodes and from said cathodes to said anodes, respectively, said forward flow of current being carried out for a time ranging from about 10 seconds to 100 seconds, at a current density of about 20 to 65 amps/sq.ft., said reverse flow of current being carried out for a time ranging from about one-fifteenth to one-fiftieth of the time employed in the forward flow of current, the reverse flow of current being maintained at a current density level of 15 to 65 percent less than the current density corresponding to the forward flow of current, and maintaining said periodic application of the current to said system until a heavy deposit of high purity copper is obtained on said cathodes.
 5. The copper electro-refining process of claim 4, wherein the cathode metal is selected from the group consisting of titanium, zirconium and stainless steel, wherein the time period during the forward flow of current ranges from about 15 to 55 seconds at a current density of about 25 to 45 amps/sq.ft., wherein the time period during the reverse flow current is about one-eighteenth to one-twenty second of the time period employed in the forward flow of current, and wherein the current density of the reverse flow of current is 25 to 45 percent less than the current density corresponding to the forward flow of current. 